The invention relates to inspection of microvias in circuit boards.
In recent years there has been a very rapid move towards increased miniaturisation of circuits because of demands of industries such as the mobile telecommunications industry. Where multi-layer circuit boards are used there id frequent use of xe2x80x9cblindxe2x80x9d vias which do not extend fully through the board but instead penetrate part of the depth of the board to reach an internal layer to make an electrical connection. Whether the board is multi-layered or not, there is also frequent use of through vias to make connections from the front surface to the back surface.
With increased miniaturisation and higher circuit densities, the sizes of the vias has been decreasing and they are now generally referred to as xe2x80x9cmicroviasxe2x80x9d. Typically, the sizes range between 20 xcexcm and 75 xcexcm. The technologies to drill microvias involve use of high-precision laser beams. The laser may be of the CO2 or UVYAG types.
While these technologies are quite effective, in full-scale production various defects may arise due to problems such incorrect setting of drilling equipment, loading of incorrect files, incorrect handling of the circuit board, of residual debris in or near the microvia. When defects arise the board panels are often too expensive to scrap and expensive re-work is required.
Existing inspection systems have been developed for inspection of components and solder paste and are not sufficiently effective for detection of defective microvias. As a result, defective microvias are often not detected until full functional tests are carried out downstream. This is a very expensive way to detect such defects. Also, in some cases the defects go un-noticed until a fault arises in the field.
It is an object of the invention to provide an inspection system for detection of defective microvias in an effective manner in a full-scale production environment.
According to the invention, there is provided a microvia inspection system comprising:
means for supporting a circuit board in a circuit board plane;
a top lighting means comprising means for illuminating the top of a circuit board at an angle of greater than 50xc2x0 to the circuit board plane;
a camera;
a controller comprising:
means for reading circuit board target data to determine target locations and sizes of microvias, and for controlling the top lighting means and the camera to capture images of said microvias, and
image processing means for determining quality of the microvias according to intensity and distribution of reflected light in the images.
In one embodiment, the image processing means comprises means for recognising a blind microvia as defective if a measured area of reflected light is below a threshold area.
In one embodiment, the image processing means comprises means for recognising a blind microvia as defective if a measured centroid of the reflected light is offset from a target location to an extent greater than a threshold distance.
In another embodiment, the image processing means comprises means for recognising a microvia as defective because of under-drilling if the measured area of reflected light is lower that a threshold area and the centroid is within a threshold distance from a target location.
In a further embodiment, the image processing means comprises means for recognising a blind microvia as over-drilled or blocked with residual debris if the measured area of reflected light is less than a threshold area and the centroid is greater than a threshold distance from a target location.
Preferably, the camera is telecentric.
In one embodiment, the top lighting means comprises means for generating on-axis or near-on-axis illumination.
In another embodiment, the top lighting means comprises a projection lens for the camera for on-axis top illumination.
In one embodiment, the camera resolution is sufficient for less than 16 xcexcm object pixel size.
In one embodiment, the robotic system and the board support means comprise means for maintaining a fixed working distance for all measurements.
In another embodiment, the top lighting means comprises means for emitting light in the visible wavelength range.
In one embodiment, the top lighting means further comprises a low-level lighting means for directing light at an angle of less than 40xc2x0 to the circuit board plane, and the controller comprises means for capturing separate high-level and low-level lighting images and for using the low-level image to define a microvia boundary within which the pixels of the high-level image are processed to determine if the microvia is defective.
In one embodiment, the system further comprises a back lighting means for illuminating a circuit board from the opposite side to that of the camera, and the image processing means comprises means for determining quality of through-microvias according to the location and pattern of light passing through said microvias.
In a further embodiment, the back lighting means comprises an array of LEDs mounted in the circuit board support means to illuminate a fill work space.
In one embodiment, the back lighting means comprises in excess of 4000 LEDs.
In one embodiment, the back lighting means comprises means for activating subsets of LEDs to illuminate the current camera field of view.
In a further embodiment, the board support means comprises a transparent plate, and means for pressing a circuit board against the plate.
In another embodiment, the transparent plate is mounted at a fixed location whereby pressing a circuit board against the transparent plate provides a fixed working distance between the camera and the circuit board.
In one embodiment, the board support means comprises a transparent conveyer mounted beneath the transparent plate, a back lighting means mounted beneath the conveyor, and means for moving the back lighting means upwardly to press against the conveyor so that a circuit board is pressed against the transparent plate.
In a further embodiment, the system comprises a plurality of cameras, and the controller comprises means for controlling capture of images arising from top lighting in one camera and images arising from back lighting in another camera.
In one embodiment, the camera for capturing images from top lighting has a higher resolution than that for capturing images from back lighting.
In a further embodiment, the system comprises a beam splitter mounted to direct light to different cameras.